In a case where an error such as noise contamination happens when data is read via communications over a network or from a memory medium, it is detected from a redundant code preliminarily added to the data whether or not any error occurs. If there is an error, error correction is conventionally carried out so as to reproduce original data. However, in cases where a given data amount includes many errors, the data cannot be reconstructed properly. For example, there are some cases where a recording device, such as a hard disk or an optical disk apparatus, cannot carry out the error correction with respect to a storage medium because the storage medium is damaged due to an impact or the like. Especially, in a case where the storage medium is an optical disk, a recording surface thereof is touchable by hand directly. The direct touch to the recording surface may possibly cause so big a damage on the storage medium that the error correction cannot be performed on the storage medium. Further, in some cases, the optical disk itself deteriorates so much due to direct or indirect sunlight that the error correction cannot be performed on the storage medium.
There is a well-known technique for preventing such data deterioration or data loss. The technique is called RAID (Redundant Arrays of Inexpensive Disks). The RAID is a technique in which (i) original data to be recorded is split into a plurality of pieces of sub data, and (ii) the plurality of pieces of sub data are recorded into a plurality of hard disks, respectively, in a distributed manner. At the time when the pieces of sub data are recorded into to the respective plurality of hard disks in a distributed manner, parity information is added to the pieces of sub data. With this arrangement, even if reading of data from any of the plurality of hard disks is failed, it is still possible to reconstruct the original data properly based on the other piece(s) of data read from the other hard disk(s).
Other than the technique for recording data into a plurality of hard disks in a distributed manner, there is disclosed another technique in which a single hard disk is arranged in the following manner so as to a realize mirroring function (Patent Literature 1). More specifically, the technique disclosed in Patent Literature 1 is so arranged that data recording is carried out such that data is written into 2 portions in a data storage medium, and data reading is carried out such that the data is read from either one of the 2 portions. With the arrangement, even if either one of the pieces of data causes an error, the other one of the pieces of data can be read.
Further, as still another technique, there is disclosed an information processing apparatus for performing data recording on a single optical disk including a plurality of recording layers in such a manner that data and its backup data for reconstructing the data are recorded into respective recording layers among the plurality of recording layers or individually recorded into the same layer (Patent Literature 2). The information processing apparatus arranged as such can achieve an improvement in efficiency of recording data and its backup data.
However, with the conventional arrangement, it is difficult to increase reliability of reconstruction of lost data in the optical disk in which data has been recorded into the plurality of recording layers.
More specifically, the optical disk in which data is recorded into a plurality of layers is arranged such that in a case where respective pieces of data are read from or recorded into different recording layers (for example, a recording layer 100 and a recording layer 200), light emitted from an optical head toward the optical disk is focused on a target layer where data is to be read from or recorded in (see FIG. 23 and FIG. 24). FIG. 23 and FIG. 24 show the conventional technique, each illustrating how light from the optical head is irradiated to a recording layer (100, 200) at the time of data recording or data reading.
As illustrated in FIG. 25, when a protection layer 300 is partially scratched (damaged) such that the scratch gets in the way of the light that is irradiated to the recording layer 100 in which a target piece of data is recorded, there may arise such a problem that the piece of data cannot be read from the recording layer 100 due to the scratch. In addition, in a case where (i) another piece of data is recorded in the recording layer 200 and (ii) recording positions (where data is recorded) of the recording layers 100 and 200 overlap each other in a light-irradiation direction of the light from the optical head, there may occur such a problem that the another piece of data cannot be read from the recording layer 200, either (see FIG. 26). FIG. 25 and FIG. 26 show the conventional technique, each illustrating (i) data recorded in a recording layer (100, 200) and (ii) how light from an optical head is irradiated to the recording layer, in a case where a protection layer (300) of an optical disk is damaged.
That is, in the optical disk in which data is recordable in a plurality of layers, there are some cases where not only data recorded in a given recording area of the recording layer 100 but also data recorded in a given recording area of the recording layer 200 may become unreadable due to the scratch on the protection layer 300 (see FIG. 27). On this account, in a case where data and its reconstruction data for reconstructing the data are recorded in the recording layer 100 and the recording layer 200, respectively, in a distributed manner, both the data and the reconstruction data may become unreadable due to a single scratch on the protection layer 300, in some cases. Here, FIG. 27 illustrates a relationship between (i) how light from an optical head is irradiated, (ii) the scratch on the protection layer 300, and (iii) ranges in the recording layers 100 and 200 in which the scratch affects data writing and data reading.
Patent Literature 1 described above does not improve the reliability of reconstruction of lost data in the optical disk in which data is recordable in a plurality of layers. Further, Patent Literature 2 does not prevent the problem that both of the data and the reconstruction data become unreadable due to the single scratch on the protection layer 300.